Non‐isothermal rheological response in melt spinning and idealized elongational flow

Almost all polymer processing operations involve moving and shaping the polymer as a melt and then cooling it, usually quite rapidly, to a solid state. In order to develop models for such processes we have begun systematic studies in non‐isothermal rheology; here we interpret the results in the context of melt spinning. Theoretical predictions of stress vs distance from the spinneret were calculated from generalized (non‐isothermal) viscoelastic theory and compared with Dees' melt spinning data on high density polyethylene. Despite certain experimental and theoretical difficulties, the agreement is good. Surprisingly, an additional theoretical curve, based on a simple Trouton viscosity, also gave a reasonable approximation over much of the distance, despite the transient nature of the flow. To understand this phenomenon further, a more tractable theoretical problem was studied in detail; the problem, of constant elongational flow (Ė = constant) in the presence of a constant rate of temperature change (dT/dt = constant). The results depend on two dimensionless groupings; the first is the usual product of a relaxation time and Ė; the second involves the ratio of normalized dT/dt to Ė. When the second group is large, a quasi‐viscous state exists. The melt spinning data for the HDPE may be near this state.